This invention relates generally as indicated to fluid control valves with angled metering ports and more particularly to such control valves in which the conventional valve flow force gains that normally occur upon opening of the valves are compensated for by utilizing angled metering ports in the valve sleeve which direct the return or high pressure fluid flow against the valve flow metering edges in a direction producing an opening reaction force in opposition to such conventional valve flow force gains.
It is well known that in spool-type valves with square edged lands, and in other types of sliding valves, steady-state forces substantially proportional to the flow rates through the valves are generated within the metering chambers and act on the moving members in a direction that tend to close the valves. Maintaining control over these axial closing forces is especially important, for example, in high performance valves where it is desirable to operate the valves throughout their entire operating range by means of relatively small control forces such as may be generated by direct drive force motors and the like.
Heretofore, various valves have been designed to compensate for such flow force gains. One such valve design is disclosed in U.S. Pat. No. 2,964,023, which is assigned to the same assignee as the present application. As shown, the valve spool of such patent is provided with two groups of small diameter inclined passages, one group of which is associated with each of the fluid return grooves. The inclined passages are arranged such that more passages are progressively moved into communication with the respective return grooves as the valve spool is moved to positions of greater displacement from the nuetral position. As disclosed, by progressively opening the small diameter passages, a decentering force proportional to the centering force is provided. Such a valve construction has the drawback that the relative size and location of the passages must be carefully selected in order to obtain the desired decentering force. The passages may either be formed in the valve spool or the valve sleeve to provide the desired fluid return flow path from the control ports to the reservoir return ports.
It is also known to provide a series of small radial holes arranged in a spiral around a valve sleeve on the supply side of the metering chamber to direct the high pressure fluid into such chamber at substantially 90.degree. so that the flow forces developed on the valve spool are substantially reduced. Here again the relative size and location of the spiral holes must be carefully chosen on the basis of the required sensitivity of the valve. Moreover, with such an arrangement, there is no compensation of the valve centering force during the initial opening of the valve. Some compensation of the valve centering force has been obtained sooner by placing the first one or two holes in the valve sleeve at an angle of approximately 30.degree. as measured from the valve axis so that the jet streams from these holes will impinge on a face of the valve causing an opening reaction force. However, this latter construction still does not provide for any reduction in the valve centering force during the initial opening of the valve.
Flow force compensation may also be obtained by providing negative flow force contours or recirculation lands on the valve spool return edge in series with the positive flow induced force on the pressure control edge. However, such recirculation lands provide very little compensation for conventional valve flow force gains during initial valve opening and low flows unless partial annular openings, for example, in the form of wedge cuts are provided on the flow metering edges in combination with shaped geometrical contours on the valve spool as disclosed, for example, in U.S. Pat. No. 4,155,535. In either case, the formation of these valve spool contours greatly increases the cost of manufacture of such valves.